In recent years, by rapid technical-development in electronics field, downsizing and lightening of electronic apparatuses progress. As the result, electronic apparatuses are becoming more and more portable and cordless, and downsizing, lightening and improvement in energy density of secondary power sources as driving sources of electronic apparatuses are strongly desired. In order to respond to such demands, lithium secondary batteries which can exhibit a high energy density have been developed. Lately, ultrathin and lightweight secondary batteries using a laminate film including aluminum as container members have also been developed and commercialized.
However, the thickness of such a secondary battery using the laminate film including aluminum changes due to volume expansion and shrinkage of electrodes when charge and discharge. The change in thickness causes a twisted battery, or making a distance between electrodes larger, leading to larger resistances of the battery, and finally reduced performance of battery.
In order to solve such a problem, batteries using lithium titanate as a negative electrode active material have been proposed. Lithium titanate exhibits almost no volumetric change associated with charge and discharge, and extremely small changes in the thickness of the battery. Thus, by using lithium titanate as a negative electrode active material, the above described problem can be solved.
In the lithium titanate, the inserting-and-extracting reaction of lithium progresses at approximately 1.55 V (vs. Li/Li+). The battery using the lithium titanate in the negative electrode and, for example, LiMn2O4 in a positive electrode exhibits approximately 2.5 V of a battery voltage. This voltage is much lower than 3.9 V, which is the battery voltage of widely commercialized lithium-ion secondary batteries including carbon negative electrode and LiMn2O4 positive electrode. That is, an increase in series-connecting number (number of batteries) is needed for a battery module or a battery pack including nonaqueous electrolyte batteries containing lithium titanate in order to realize the same voltage as that of a battery module or a battery pack including the carbon negative electrode and the LiMn2O4 positive electrode.